The problem: Without taking your pen off the paper, draw four straight lines such that all nine dots are intersected.
The answer: Draw from 'A' to 'B' to 'C' to 'D' to 'B'. See below.
Want to get your own back on the person who asked you this? Show them the diagram below.
Now challenge then to join the dots, as above, but using only three lines!
The answer requires them to realise that the dots are not a theoretical one dimensional representation but actually occupy some (small) area. It is therefore possible to draw lines at a slight angle, somewhat like this:-
Given a large enough piece of paper the lines will eventually converge.
Now, if they get that, or if you simply want to compound their misery, demand that they only use a single line. Be cruel and ask for two separate solutions!
Answer 1: The 'crude' option, just take a sufficiently wide paintbrush such that a single stroke obliterates all nine dots.
Answer 2. The 'cuning' option, wrap the paper around a cylinder and draw a spiral.